U.S. Pat. application Ser. No. 07/412,885 to the inventor hereof, entitled "Acoustic Digitizing System", an acoustic position locating system is disclosed which employs an acoustic point source transmitter, the structure of which is shown in FIGS. 1 and 2 hereof. The acoustic transmitter comprises a conical resonator 10 mounted on a piezoelectric actuator 12. Both resonator 10 and actuator 12 are mounted on a pedestal 14 and actuator 12 is connected via pins 16 and 18 to a pulse source 20. The transmitter structure is mounted within a housing 22 that has a face plate 24 (see FIG. 2). An opening 26 is centrally located in face plate 24 and provides a "point source" effect for acoustic emanations produced by piezoelectric actuator 12 and resonator 10. The choice of the diameter of opening 26 involves a tradeoff between emitted power and wavefront beamwidth.
As can be seen in FIG. 3, when piezoelectric actuator 12 is energized, it creates a planar acoustic wavefront 30 which impinges upon the inner surface of face plate 24. As wavefront 30 passes through opening 26, assuming opening 26 is sufficiently small, the transmitted waveform assumes an omnidirectional beam pattern 32 due to diffraction effects created by the beveled edges of opening 26.
In the environment of an acoustic position locating system, it is important that wavefront 32 be substantially omnidirectional and uniform to enable accurate position sensing. The structure shown in FIGS. 1 and 2, while accomplishing the aforementioned signal emanation characteristics, requires that the distance between conical resonator 10 and the inner surface of faceplate 24 be very accurately maintained, to maintain a uniform output amplitude wavefront characteristic. This dimension becomes even more critical when more than one transmitter is employed (e.g. on a stylus, where the placement of two transmitters allows both the position and orientation of the stylus to be determined).
Another problem with the acoustic transmitter of FIG. 1 is that piezoelectric element 12 is mounted on a pedestal 14. As a result, when actuator 12 is energized, a portion of its energy is induced into pedestal 14 and thereby is lost.
Other prior art known to the inventor hereof can be found in U.S. Pat. Nos. 4,228,379 to Guscott et al. and 4,278,851 to Takay, both of which show piezoelectric transducers for the generation of sonic signal having substantially omnidirectional wavefronts. U.S. Pat. Nos. 3,675,053 and 3,749,854, both to Mifune et al; 4,283,649 to Heinouchi, 4,456,848 to Yasuda et al.; 4,486,868 to Kodera et al.; and 4,602,245 to Yang et al. all show the use of various cone-shaped acoustic resonators in combination with piezoelectric transducers.
In U.S. Pat. Nos. 4,456,849 and 4,607,186 to Takayama et al., ultrasonic transducers are disclosed which both employ conical resonators and attached piezoelectric actuators. In these patents, however, the resonators support the piezoelectric actuator. Various faceplates are disclosed in the '186 patent for providing directional wavefronts from a horn structure. In addition, the resonators are mounted by ring shaped members of elastic or vibration absorbing substances which hold them in place against a faceplate. The ring-shaped members, used to hold the resonators substantially dampen these vibrations when energized. Such heavy dampening affects not only the output amplitude of the ultrasonic signal, but also the number of cycles thereof which are generated. While the latter is desirable, the former is not and results in a substantial decrease in the output power of the resonator.
Accordingly, it is an object of this invention to provide an improved ultrasonic transmitter.
It is another object of this invention to provide an improved ultrasonic transmitter which exhibits improved output amplitude generation characteristics.